The World Without Us

Still others point to a handful of archaeological sites that supposedly predate Clovis. Archaeologists who excavated the most famous of these, Monte Verde, in southern Chile, believe that humans may have settled there twice: once 1,000 years prior to Clovis, the other time 30,000 years ago. If so, at that time the Bering Strait would likely not have been dry land, meaning an ocean voyage from some direction was involved. Even the Atlantic has been suggested, by archaeologists who think that Clovis techniques for flaking chert resemble paleolithics that developed in France and Spain 10,000 years earlier.

Questions about the validity of Monte Verde’s radiocarbon dates soon cast doubt over initial claims that it proved early human presence in the Americas. Matters were further muddied when most of the peat bog that had preserved Monte Verde’s poles, stakes, spear points, and knotted grasses was bulldozed before other archaeologists could examine the excavation site.

Even if early humans did somehow find their way to Chile before Clovis, argues Paul Martin, their impact was brief, local, and ecologically negligible, like that of the Vikings who colonized Newfoundland before Columbus. “Where are the abundant tools, artifacts, and cave paintings that their contemporaries left all over Europe? Pre-Clovis Americans wouldn’t have met competing human cultures, like the Vikings did. Only animals. So why didn’t they spread?”

The second, more fundamental controversy about Martin’s Blitzkrieg theory, for years the most accepted explanation for the fate the of the New World’s big animals, asks how a few nomadic bands of hunter-gatherers could annihilate tens of millions of large animals. Fourteen kill sites on an entire continent hardly add up to megafaunal genocide.

Nearly half a century later, the debate Paul Martin ignited remains one of science’s greatest flash points. Careers have been built upon proving or attacking his conclusions, fueling a protracted, not-always-polite war waged by archaeologists, geologists, paleontologists, dendro- and radiochronologists, paleoecologists, and biologists. Nevertheless, nearly all are Martin’s friends, and many are his former students.

The leading alternatives they’ve proposed to his overkill theory involve either climate change or disease, and have inevitably come to be known as “over-chill” and “over-ill.” Over-chill, with the greatest number of adherents, is partly a misnomer, because both overheating and overcooling get blamed. In one argument, a sudden temperature reversal at the end of the Pleistocene, just as glaciers were melting away, plunged the world briefly back into the Ice Age and caught millions of vulnerable animals unaware. Others propose the opposite: that rising Holocene temperatures doomed furry species, because they had adapted over thousands of years to frigid conditions.

Over-ill suggests that arriving humans, or creatures that accompanied them, introduced pathogens that nothing alive in the Americas had ever encountered. It may be possible to prove this by analyzing mammoth tissues that will likely be discovered as glaciers continue to thaw. The premise has a grim analog: Most descendants of whoever were the first Americans died horribly in the century following European contact. Only a tiny fraction lost their lives to the point of a Spanish sword; the rest succumbed to Old World germs for which they had no antibodies: smallpox, measles, typhoid, and whooping cough. In Mexico alone, where an estimated 25 million Meso-Americans lived when the Spaniards first appeared, only 1 million remained 100 years later.

Even if disease mutated from humans to mammoths and the other Pleistocene giants, or passed directly from their dogs or livestock, that would still put the blame on Homo sapiens. As for over-chill, Paul Martin replies: “To quote some paleo-climate experts, ‘Climate change is redundant.’ It’s not that the climate doesn’t change, but that it changes so often.”

Ancient European sites show that Homo sapiens and Homo nean-derthalensis both drifted north or south with advancing or receding ice sheets. Megafauna, Martin says, would have done the same. “Large animals are buffered against temperature by their size. And they can migrate long distances—maybe not as far as birds, but compared to a mouse, pretty well. Since mice, pack rats, and other small, warm-blooded creatures survived the Pleistocene extinctions,” he adds, “it’s hard to believe that a sudden climate shift made life intolerable for big mammals.”